There are situations in industry where contact between two fluids exchanging heat must be prevented. For example, in nuclear power plants and in chemical plants, contact between certain fluids with exchange heat will result in unacceptable contamination. Further, there are fluids which if contact between them is permitted will cause a violent reaction. For this reason, in nuclear power plants, where sodium is used to heat water or steam it is essential that precautions be taken to prevent contact between the two fluids. The heat exchangers usually comprise one or more tube sheets which separate the primary from the secondary fluid and a number of tubes connected with the tube sheet so that one of the fluids can pass through the tubes and the other fluid over the tubes, to place the fluids in indirect heat exchange.
Since the fluids pass in closest proximity to each other at the tubes, it is common to use duplex tubes, that is, tubes having a double wall. In effect, each duplex tube is really two coaxial tubes since it has an inner wall, which is separate from the outer wall, over the area where both fluids contact the tube. With this arrangement, failure of one of the tube walls will not result in contact between the primary fluid and the secondary fluid because there will still be a wall between the fluids.
Associated with the problem of preventing leakage between the primary fluid and the secondary fluid is the problem of detecting leakage when it does occur. In a heat exchanger using duplex tubes, it is desirable to detect leakage through one of the walls of the duplex tube so that the fault can be detected before both walls fail.
It is highly desirable that the leak detection system not only indicate the presence of a leak, but also indicate where the leakage is occurring. Such a capacity assures the replacement, repair or plugging of the proper tube to thereby provide safe and efficient operation.